1. Field of the Invention
The present invention relates to a bead wire for a bead core of a tire and to a tire incorporating the same, and more particularly to a bead wire for a bead core of a tire designed to allow easy mounting of the tire on a wheel rim and also designed to resist the separation of the tire from the rim under diverse operating conditions.
2. Description of Related Art
As is known in the art, a tire generally comprises at least one carcass reinforcing ply having edges turned up around bead cores, a tread, and belts placed between the carcass and the tread. Strips of rubber filling are positioned between the sides of the carcass reinforcing ply and the upturned edges of the same reinforcing ply. The portion of the tire comprising the bead core and the rubber filling forms the tire beads, which function to anchor the tire over a corresponding mounting rim.
The wheel rim comprises a central cylindrical channel from which branch off axially and outwardly, from opposite sides, diverging surfaces each terminating in a vertical peripheral flange called the xe2x80x9crim balconyxe2x80x9d. The diverging surfaces form the bead seats for the beads of the tire.
The bead core is substantially inextensible in circumferential direction and can be formed of a single element or of several elements, such as steel wires, steel cords, and other similar components placed in an annular disposition. The individual elements are referred to as bead wires, and when assembled together they form the bead core.
The inner diameter of the tire beads substantially coincides with the diameter of the innermost surface of the bead cores, except for a difference between the two diameters caused by a thin lining of rubber. The diameters of the inner annular surfaces of the bead cores and tire beads are smaller than the diameter of the rim balcony, and are chosen so that, once the beads are forced over the balcony to their respective bead seats on the rim, they are pushed along the diverging surfaces of the bead against the inner surfaces of the flanges by the pressure of the air inside the tire.
The operations of mounting the tire onto the rim are performed according to methods well known to those skilled in the art. The operation starts by deforming the first bead of a tire into an oval configuration, so that when positioned in front of the rim with the oval aperture suitably oriented, a portion of the bead slips over the balcony of the rim. Then the rest of the bead completely slips over the rim balcony, so that the bead can then be pushed toward the bead seat. The preceding steps are then repeated for the second bead. Finally, the tire is inflated to press the beads against the internal surfaces of the rim balcony of the bead seat.
Because of the rigidity of the bead cores, mounting the tire on the rim requires application of a large force to deform the bead core from its circular configuration to an oval one, causing obvious difficulties of application for certain bead core structures, and with a risk of breakage in some circumstances if the limits of elastic deformability of the bead wires are exceeded.
Since the pressurized air in the tire is used to maintain the tire bead pressed against the inner surface of the rim flange, when the tire deflates this force is no longer applied, and the tire bead can leave its bead seat falling into the central channel of the rim. This unseating, usually, immobilizes the vehicle because the rim balcony quickly enters in contact with the pavement, making traction and control of the vehicle impossible. For example, a conventional tire is inflated at an operating pressure of 1.8-2.0 bars. When the inflation pressure falls below approximately 0.8 bars, the tire beads are likely to unseat from the bead seats. As a result, a second requirement for modern tires is to be able to remain in place on the rim even in the event of a perforation and subsequent loss of air within the tire. This requirement calls for a bead core exerting sufficient force on the rim so that the tire beads will remain seated on the rim, even in the absence of air pressure pushing them in place. This result cannot be achieved using conventional bead cores, unless humps are formed on the rim to act as a barrier preventing the beads from slipping in the center channel portion of the rim. According to the state of the art, it has not been possible to provide bead cores that exert sufficient force on the rims to maintain the tire beads on the rim when the tire deflates, and that at the same time can be stretched sufficiently to allow mounting of each tire on the rim with conventional tools.
Several types of conventional bead cores used in tire beads are known. For example, a first design provides for a bead core formed by a rubber-coated steel wire wound in a spiral to form a first layer of side-by-side coils. Subsequent layers are superposed on the first layer, and also consist of helical windings of the same wire. One known construction of this type of bead core comprises four layers of four wires each. An additional type of bead core calls for the use of several individual wires and, more precisely, of a first Wire wound in a spiral to form several coils arranged radially in a vertical plane. Subsequent wires are similarly wound in vertical planes, and are placed side by side to the first plane. One such construction known as 4xc3x974 comprises four wires disposed in four layers.
Bead cores with improved characteristics of flexibility, and consequently greater deformability without risk of breakage, are also known. One of these known structures is a spiral bead core. This bead core is formed of a central cable around which several wires are wound in a spiral. In this design, 4 or 5 groups of steel wires can be disposed in a structure having 4 or 5 layers of the steel wire groups, set up in a spiral pattern. However, the construction of this spiral bead core requires a number of separate reels for the cable and for the various wires. This results in increased manufacturing costs than, for example, those incurred in the construction of a bead core containing a single type of wire.
Another problem encountered in mounting tires is that generally, in the construction of the bead cores as well as in the construction of the wheel rims, the actual dimensions obtained often vary from their selected tolerances. In these cases, dovetailing of the tire bead over the relative bead seats can occur. This may result in breaking of the bead core when the actual diameter of the tire bead is significantly smaller than the specified dimension, or in slipping of the tire bead over the bead seat during rotation when the bead core is larger than the specified dimension.
German Patent Application DE 3829460 A1 describes a bead core made with a shape-memory material, preferably a Nixe2x80x94Ti alloy. The mounting method described in the application calls for temporarily deforming the bead core into an oval configuration, and after mounting the tire on the rim, submitting the bead core to heat treatment at the crystallization temperature of the alloy (between 65 Deg. C. and 90 Deg. C. ), so that the bead core recovers its annular shape. Thus this method provides a bead core that holds the tire beads against the rim with sufficient force while being elastic enough to allow mounting of the tire. However, the mounting of the tire according to this method requires specialized tools and heating equipment.
Known practices to resist tire unseating are generally based on modifying the surfaces of contact of both the tire beads and the bead seats. One of these practices entails using a protuberance on the base of the tire bead and an aperture on the rim designed to receive the protuberance. The protuberance of the tire bead when inserted into the aperture of the rim prevents the tire beads from separating from the rim.
The search for a suitable design for bead cores must take into consideration the ability of the bead to be deformed when the tire is mounted on a wheel rim, while also keeping the beads of the tire elastically clamped to the rim after mounting. This gives rise to a first requirement that must be satisfied by materials used in forming the bead cores, which is that the material should have the ability to be stretched to a great extent without failing. In addition, the material must be able to return to its original shape once the load has been removed, at ambient temperature and without retaining the effects from the previous stretching.
These requirements are made even more complex and urgent by the additional need to make tires that can travel tens of kilometers after going flat, without the tire separating from the rim. Ideally, a flat tire should be able to provide a safe ride for the driver and passengers of a vehicle to reach a garage where it can be changed, possibly after traveling distances of at least 50 to 100 km, at speeds approximately up to 80 km/hr.
The desired safety features require that the tire beads of a flat tire remain in the respective bead seats, even with no air pressure pushing on the inner surfaces of the tire beads. The tire beads must be prevented from slipping inwards toward the smaller rim diameter of the central cylindrical channel, causing unseating of the tire beads when transverse forces generated by a turn are present.
In view of the aforementioned state of the art, the Applicant realized that to obtain an acceptable solution without technical and/or economic drawbacks, it was necessary to keep the parts of the tire and rim involved in the mounting phase at ambient temperature. It was also necessary to use standard parts at the interface of the rim and the tire, including the parts responsible for keeping the deflated tire attached to the rim. This precluded using mechanical couplings to resist unseating of the tire.
Applicant also realized that an optimal solution could result from the harnessing of the very conditions that arise specifically in a deflated tire, seeking where possible to use the resulting phenomena to counter the risk of unseating. It has been observed that under deflation conditions, the sides of the tire fold and bring the two resulting parts of the folded sidewall into contact with each other, causing rubbing, which causes heating of the entire folded area all the way to the bead cores.
In determining an optimal solution, the Applicant realized that the effect of a temperature increase, being transmitted to the bead core, could cause a reaction inside the bead core if it were made of a material chosen so as to respond to a temperature increase. It was also understood that an optimal solution to the problem might include the use of a bead core capable of contracting increasingly in the presence of temperature increases caused by the deflation condition. The use of such a solution would make it possible to keep the bead core in the bead seat, even with no air pressure present in the tire. The solution, however, also require that the tire bead core has to be easily deformable for mounting onto the rim, and progressively contractible in the state of deflation. It was found that precisely, because of the conflicting demands of opposite deformability of the bead core at different temperature conditions, it was possible to solve the problem by forming a bead core comprising a material with characteristics of superelasticity and/or shape memory, preferably both. The entire bead core, or only a circular portion of it could be formed of the material.
As is known and as used herein, a material with characteristics of xe2x80x9csuperelasticityxe2x80x9d is a material that can be progressively deformed to a high degree by applying a constant load and maintaining a constant temperature. The material then recovers from the deformation suffered when the load is removed and the temperature is left unchanged. Moreover, the material used also has properties of xe2x80x9cshape memoryxe2x80x9d in the sense that when the material is subject to heating between two predetermined temperatures in conditions of mechanical constraint, it undergoes a transformation from a first structure to a second structure, producing a stress opposed by the constraints. The practical result is that a contraction force is exerted by the material on the constraints, when the temperature is raised beyond a preselected value.
The invention is thus a bead core for tires designed to allow easy assembly of the tire on a wheel rim by undergoing large elastic elongations to accommodate the dimensions of the rim, and at the same time capable of resisting the separation of the tire bead from the rim while driving after the tire has been punctured. This is achieved by providing at least one annular portion of the bead core which has both properties of superelasticity and of shape memory, so that it can react to changes in the tire operating conditions, and does not require specialized tools to mount the tire.
A first aspect of the invention is thus a metallic bead core for a tire bead designed to be applied to a wheel rim, the bead core being formed from a material having characteristics of superelasticity and/or of shape memory.
The characteristics of superelasticity allow the bead core to deform at ambient temperature, producing increasing elongations for a constant load. This is due to a change in the structure of the superelastic material due to the loading. This load is defined as the critical load of transformation. The bead can thus slip over the balcony of the wheel rim without causing heavy load stresses on the material. Subsequent removal of the load at ambient temperature allows recovery from the deformation, while the gripping pressure exerted by the tire beads on the bead seats is maintained.
Furthermore, the characteristics of shape memory of the bead core are such as to generate, during operation of the tire, forces of contraction which maintain the beads in the bead seats. These forces of contraction increase as a function of the heating to which the bead core is subjected, because the structure of the shape memory material changes as a result of a change in temperature. The described forces of contraction are particularly useful in increasing the grip of the beads on the rim, keeping the tire from unseating when in a deflated condition.
Preferably, the bead core is formed by a material having characteristics which comprise:
a) a temperature As less than Ta
b) a temperature Af less than Tmax 
where:
As is the temperature of transformation of the material corresponding to the passage from a martensitic structure to the start of the formation of an austenitic structure;
Af is the temperature corresponding to the complete transformation of the material structure into austenite;
Ta is the ambient temperature;
Tmax is the predetermined temperature corresponding to the maximum temperature reached by the tire using the present bead core, while operating in a deflated condition.
A second aspect of the invention consists of a tire having at least one toroidal carcass and one tread, the carcass presenting a central crown and two sidewalls terminating in a pair of annular beads, each incorporating a metallic bead core for anchoring the bead to a corresponding mounting rim. The tire is characterized by having at least one bead comprising a bead core made of a superelastic and shape memory material, which allows increasing deformations of the bead core when subject to a mounting load of constant magnitude at ambient temperature, and which recovers the original circular dimension of the bead core once the load ceases.
Preferably, the tire includes at least one portion of the bead core material having superelasticity and shape memory characteristics, which increase the force of inward contraction of the bead core when a predetermined temperature is reached. The contraction force develops as the tire operates while deflated, and increases the gripping force of the bead on the rim.
Another aspect of the invention is a method for mounting a tire to a rim having a balcony portion, having the steps of forming a tire with bead cores comprising at least one portion made of superelastic and shape memory alloy, and successively stretching each bead core by applying a constant load so they will slide over the balcony portion of the rim. The constant load is then released to allow the bead wire to return to a length corresponding to the rim diameter, and the tire is inflated.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.